The Siwalik fossil record of the Indian subcontinent documents ecosystem changes under different configurations of geodynamically controlled barriers and climatic change. Over the Neogene, the Siwalik faunal province, which extended from Pakistan to Myanmar, was either isolated or connected to Eurasia depending on sea level and montane barriers. Four biogeographic modes result from the interaction of permeable versus impermeable barriers and changing versus stable climates. Each mode corresponds to a unique set of macroevolutionary predictions for vertebrate faunas. For example, the combination of permeable barriers and climatic stability should result in low rates of immigration, speciation, and extinction, and stable ecological structure of Siwalik vertebrate faunas. Three of the four modes are present within the Siwalik sequence of northern Pakistan.
We analyzed the diversification history of Siwalik mammals from 18.0 to 5.5 Ma based on fossil collections from northern Pakistan and evaluated whether periods of faunal change or stability correspond to the predictions for three biogeographic modes. We calculated confidence intervals on species durations and then analyzed per-capita rates of origination, extinction, diversification, and turnover per 0.5-myr intervals for small mammals (<1 kg) and large mammals (>1 kg) separately. During two intervals of permeable barriers and changing climatic conditions, both small mammals and large mammals showed similar patterns of diversification, with a middle Miocene interval dominated by high origination rates and positive diversification and a late Miocene interval dominated by high extinction rates and negative diversification. During a middle Miocene interval of impermeable barriers and stable climate, large mammals exhibited stable diversity and small mammals showed modest turnover without significant change in diversity. During a late Miocene interval of permeable boundaries and stable climate, large mammals showed significant turnover but little change in diversity, whereas small mammals exhibited stable diversity. These patterns suggest that changing climatic conditions affect small mammals and large mammals in a similar manner via geographic-range shifts. In contrast, under stable climatic conditions, small mammals and large mammals showed non-synchronous turnovers and stable diversity. These contrasting patterns of faunal change and stability support the concept of multiple biogeographic modes of biotic change.
Host: Professor John Vandermeer